1. Field of the Invention
The present invention relates to a method for manufacturing a diffraction type optical element for selectively dividing and diffracting spectral light into a specific direction, or having asymmetric convexities in the cross-section thereof.
2. Description of the Related Art
As a diffraction type optical element, there are known various optical elements, such as a diffraction grating, a hologram or a photonic crystal, etc. These elements are installed for instance in a photo-coupler, a polariscope, a wave divider, a wave-length filter, a reflector, a mirror of high efficiency wave-guide type, a double reflection (birefringence) element, a light scattering body, or a mode converter, etc.
These optical elements, such as the diffraction grating and so on, are fundamental optical parts which are widely used in various fields of optical technology, including optical measurement and optical communication. An optical element of the diffraction type which is commercially available is generally manufactured by the so-called stamper method or by a coherent exposure method.
In the stamper method, the diffraction type optical element is formed by a process in which an organic material is injected into a mold of an original and is transcribed onto a substrate.
On the other hand, in the coherent exposure method, there is already known a method, in which an organic light sensitive material pasted on the surface of a substrate is exposed to coherent light by irradiating light onto both sides of the film surface of the pasted light sensitive organic material and the substrate surface, thereby controlling the configuration of a cross-section of the diffraction grating. Additionally, there is also a method in which two light beams are irradiated upon the film surface at different incident angles respectively ("Optical Integrated Circuit", under the joint authorship of Hiroshi Nishihara, Masamitsu Haruna, Toshiaki Sakakibara, Ohm Co., 1985, Chapter 7).
However, the element, on the surface of which is evaporated a high reflection material such as a metal, is sometimes used when it is in the process of being applied.
Also, since a glass material has superior characteristics with respect to flatness, processing accuracy, weather resistance, etc., there is already known a diffraction grating for use in the field of optical communication, which is formed directly on the surface of the glass substrate by a micro processing.
In treating such micro processing on the glass substrate, conventionally there is known a wet etching (chemical etching) method using an etchant including hydrofluoric acid, etc., or a dry etching (physical etching) method including, for example reactive ion etching.
For some time, for instance, for selectively dividing and diffracting spectral light into a specific direction, a diffraction grating having a configuration which is continuous from a symmetric sinusoidal wave form to an asymmetric curved configuration in cross-section of the convex on the surface, or which has a configuration of continuous asymmetric trapezoids, or a saw-toothed or a diffraction grating, has been well known.
In the case of manufacturing the diffraction type optical element having such an asymmetric cross-sectional configuration, especially in using the stamper method among the conventional methods mentioned above, there are included steps which require a high accuracy in positioning and a relatively long work time, including for example a pattern transcription process and a baking process, and it is difficult to increase the processing accuracy of a master up to a certain level sufficient to be able to control the micro configuration.
On the other hand, in the case of using the coherent exposure method, since the thin film is made of an organic high polymer, it is inferior with respect to the characteristics of weather resistance and of heat resistance.
Further, in the case of implementing the reactive ion etching method, or the chemical etching method of dipping in a solution, there is included a process of lithography for producing a mask, in which process are included many processes, such as painting of the organic material, drying, exposure, baking, development, etc. And, for changing or modifying the cross-sectional configuration of the diffraction grating, for instance, for obtaining a diffraction grating having a step-like cross-section, it can be realized for the first time by repeating the exposure process and the etching process several times and by gradually changing the configuration of the mask, thereby requiring many steps.